The present invention relates to novel cyclic tetrapeptide derivatives or pharmaceutically acceptable salts thereof, application of said compounds as histone deacetylase inhibitors and MHC class-I molecule expression-promoting agents, as well as pharmaceutical compositions that comprise said cyclic tetrapeptide derivatives or pharmaceutically acceptable salts thereof as active ingredients and which have utility as pharmaceuticals such as anti-cancer agents by taking advantage of the above histone deacetylase-inhibiting or MHC class-I molecule expression-promoting action.
An individual""s own tissue cells express on their cell surface an MHC class-I molecule as an antigen presenting molecule to discriminate externally invading foreign matters and pathogens from themselves, in order to prevent false damage by their immunocytes. The immune system recognizes the MHC class-I molecule to identify the self tissue cells and eliminate them from the target of its attack. On the other hand, cancerized cells or cells infected with cancer viruses, which are originally self cells, differ from normal self cells in that they produce proteins associated with canceration or proteins derived from the cancer viruses, and antigens derived from these non-self proteins are presented by the MHC class-I molecule. The immunocytes, in particular cytotoxic T cells (CTLs), can recognize the non-self protein-derived antigens, thereby eliminating the cancer cells or cancer virus-infected cells.
It has been reported, however, that in certain kinds of cancer cells or cancer virus-infected cells, the expression of the MHC class-I molecule is reduced, so that the above elimination mechanism by the immune system is circumvented, causing expansion and enlargement of cancerized tissues as well as prolonged sustention and enlargement of cancer virus infection. In the studies for the purpose of preventing tumorization of the cancerized cells or cancer virus-infected cells, some results have been reported suggesting that therapeutic effects may be attained by recovery of the reduced expression of the MHC class-I molecule. For example, Tanaka et al. reported that in cancer cells transformed with adenovirus 12 or spontaneous melanoma, tumorization of these cancer cells disappeared upon enhancing the reduced expression of the MHC class-I molecule through introduction of MHC class-I gene; see, e.g., Tanaka, K., Isselbacher, K. J., Khoury, G. and Jay, G., Science, 228, 26-30, 1985; Tanaka, K., Gorelik, E., Watanabe, M., Hozumi, N. and Jay, G., Mol. Cell. Biol., 8, 1857-1861, 1988.
The expression of MHC class-I molecule occurs during the differentiation process after the growth of the self tissue cells, and the expression of MHC class-I molecule is expected to be enhanced by promoting the translation of endogenous proteins in this process. While there are several mechanisms that control the translation of endogenous proteins, one of those that may be considered to play an important role in gene expression is acetylation of histone proteins contained in the nuclear gene chromatins as their structural proteins. Illustratively, chromatin is composed of the basic unit referred to as a nucleosome structure, in which a gene DNA is wound around four core histone octamers. Further, the basic units form a higher-order structure. The neighborhood of the N-terminal of the core histone is in the form of a tail rich in basic amino acids and it further encloses the DNA on the above nucleosome. Lysine residues in the neighborhood of the tail region undergo reversible metabolic turnover of acetylation and are said to be closely involved in the structural control of nucleosome itself or in the transcriptional control through the control of binding with other proteins acting on gene DNA, such as transcriptional factors, silencer proteins and RNA polymerases.
As a demonstration of gene expression control depending on acetylation of histone, it has been reported that higher acetylation of histone promotes the induced expression from genes present in a region of interest while deacetylation forms a transcriptionally inactive region called heterochromatin. That is to say, histone which is a structural protein of chromatin and its acetylation are extended over the whole region of the chromosomal gene; nevertheless, it has been suggested that the function of hi stone greatly affects the expression of a specific gene and, in other words, is involved in the strict control of nuclear signal transmission. An enzyme for acetylating histone is histone acetyltransferase while an enzyme for deacetylating histone is histone deacetylase; these enzymes regulate the kinetic metabolic turnover relating to the level of histone acetylation.
If the action of histone deacetylase is enhanced, proper differentiation of cells or normalization of their morphology is inhibited; however, when the enzyme activity of the histone deacetylase is inhibited, the deacetylation from histone is inhibited and, as a result, high acetylation of histone is caused to induce the gene expression required for differentiation and normalization of cell morphology. This phenomenon has been confirmed to some extent by studies using trichostatin A shown in FIG. 1 or trapoxin analogs shown in FIG. 2, which are enzyme inhibitors against histone deacetylase. In addition, when these inhibitors are allowed to act on cells at higher concentrations, cell cycle inhibition is caused and consequently growth inhibition occurs. Trichostatin A exhibits a non-competitive enzyme-inhibiting action at low concentrations and is a reversible inhibitor; on the other hand, trapoxin analogs exhibit competitive inhibitory actions but are irreversible inhibitors. Further, it has also been reported that enzymatically active subunits of human histone deacetylase were purified on an affinity column using K-trap of a cyclic tetrapeptide compound similar to trapoxin; thus, strong evidence has been given to demonstrate that the cyclic tetrapeptide structure as found in trapoxin and the like forms a selective intermolecular linkage with said enzymatically active subunit.
As stated above, since an enzyme inhibitor against histone deacetylase can be a drug causing cell differentiation or normal morphogenesis, it may also exhibit a promotion of the expression of MHC class-I molecule that occurs as a step in the process of differentiation; however, no report confirming this possibility has been made to date. Accordingly, there is a strong need for search and proposal of histone deacetylase enzyme inhibitors that exhibit promoting actions on the expression of MHC class-I molecule in self tissue cells. Further, as stated above, a histone deacetylase enzyme inhibitor at a high concentration causes the inhibition of cell cycle and consequently exhibits growth-inhibiting action, so a need exists for the proposal of a novel anti-cancer agent that is based on the promotion of the MHC class-I molecule expression and which exhibits a combined anti-cancer action due to the contributions of not only the inhibition of tumorization and the elimination of cancer cells by immune system, but also the cell growth-inhibiting action, all being associated with the promotion of MHC class-I molecule expression.
The present invention provides a novel histone deacetylase enzyme inhibitor exhibiting a promoting action on the expression of MHC class-I molecule in self tissue cells. The present invention also provides a pharmaceutical composition comprising the histone deacetylase enzyme inhibitor of the invention as an active ingredient.
The present invention provides a cyclic tetrapeptide derivative represented by the following general formula (I), (Ixe2x80x2), (Ixe2x80x3) or (Ixe2x80x2xe2x80x3) or a pharmaceutically acceptable salt thereof: 
wherein each of R11, R12, R21, and R22 independently denotes hydrogen, a linear C1-C6-alkyl group to which a non-aromatic cycloalkyl group or an optionally substituted aromatic ring may be attached, or a branched C3-C6-alkyl group to which a non-aromatic cycloalkyl group or an optionally substituted aromatic ring may be attached; and
each of R1, R2 and R3 independently denotes a linear C1-C5-alkylene group which may have a C1-C6 side chain, in which the side chain may form a condensed ring structure on the alkylene chain;
provided that at least one of R11, R12, R21 and R22 in general formula (Ixe2x80x2xe2x80x3) is a cyclohexyl methyl group.
The present invention also provides a histone deacetylase inhibitor and an MHC class-I molecule expression-promoting agent, each comprising the cyclic tetrapeptide derivative or pharmaceutically acceptable salt thereof as an active ingredient.
The invention provides a method of inhibiting a histone deacetylase comprising administering to a subject in need thereof a cyclic tetrapeptide derivative of the invention, or a pharmaceutically acceptable salt thereof, thereby inhibiting a histone deacetylase inhibitor.
The invention provides a method of promoting an expression of an MHC class I molecule comprising administering to a subject in need thereof a cyclic tetrapeptide derivative of the invention, or pharmaceutically acceptable salt thereof, thereby promoting an expression of an MHC class I molecule.
Further, the present invention provides a pharmaceutical composition comprising the cyclic tetrapeptide derivative or pharmaceutically acceptable salt thereof as an active ingredient. In one aspect of the invention, this pharmaceutical composition is used as an anti-cancer agent.
This specification includes part or all of the contents as disclosed in the specification and/or drawings of Japanese Patent Application No. 11-53851, which is a priority document of the present application.